starknet_transaction_prover: document observability surface in README

[avi-starkware]

Adds a top-level Observability section to the README covering the
HTTP-only side endpoints (/health, /metrics), the metric names and
label sets, request-id propagation rules, the per-request log shape, the
startup banner, and the graceful-shutdown / panic-hook contract.

Updates the Environment variables table with LOG_FORMAT and
HEALTH_MAX_SATURATED_MS, the JSON config table with
health_max_saturated_ms, and the Logging subsection with the JSON
format option and URL redaction note.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

[cursor]

PR Summary

Low Risk
Documentation-only changes to README; no runtime or API behavior modified.

Overview
Documents the transaction prover’s observability surface in README.md so operators and reviewers can run it without reading implementation code.

The API Reference now points to GET /health and GET /metrics beside the JSON-RPC spec. Configuration gains LOG_FORMAT, HEALTH_MAX_SATURATED_MS, and JSON health_max_saturated_ms; the Logging section covers JSON logs and URL credential redaction.

A new Observability section describes /health (200 vs 503 when saturated, tied to -32005 and HEALTH_MAX_SATURATED_MS), /metrics (Prometheus names, bounded labels, probe bypass), x-request-id rules (including intentional dual IDs for OHTTP), the http_request log shape, startup banner redaction, and graceful shutdown plus panic-hook behavior.

^{Reviewed by Cursor Bugbot for commit c77dae0. Bugbot is set up for automated code reviews on this repo. Configure here.}

[reviewable-StarkWare]

This change is 

[avi-starkware]

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Learn more

• starknet_transaction_prover: document observability surface in README #14203 👈 (View in Graphite)
• starknet_transaction_prover: origin-level breadcrumbs for validation errors #14172
• starknet_transaction_prover: /health returns 503 when service is saturated #14171
• starknet_transaction_prover: bump prover_panics_total on panic hook fire #14170
• starknet_transaction_prover: HTTP request count + latency + in-flight metrics #14169
• starknet_transaction_prover: proving-job duration + outcome metrics #14168
• starknet_transaction_prover: Prometheus /metrics endpoint with build_info #14167
• starknet_transaction_prover: global panic hook + graceful SIGTERM shutdown #14166
• starknet_transaction_prover,tower_ohttp: OHTTP-unlinkable request-id for decapsulated content #14222
• starknet_transaction_prover: per-request structured log with request-id #14165
• starknet_transaction_prover: redact URL credentials in logs and add startup banner #14164
• starknet_transaction_prover: add /health endpoint via HealthLayer #14163
• starknet_transaction_prover: add --log-format text|json CLI flag #14162
• main

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